1. Field of the Invention
The present invention relates to an optical signal converter, an optical encoder, an optical decoder, an optical code division multiplexing communication apparatus, and the like for use in optical code division multiplexing communications.
2. Description of the Related Art
In recent years, the communication traffic demands have been rapidly increased due to the widespread proliferation of the Internet. The optical multiplexing communication technologies have been so far developed from an optical time domain multiplexing (OTDM) communication scheme to a wavelength division multiplexing (WDM) communication scheme to increase the communication capacities. It is an optical code division multiplexing (OCDM) communication scheme which is expected as the next generation optical multiplexing communication scheme. The OCDM communication scheme is characterized by the capability to set a plurality of communication channels in the same time slot and on the same wavelength. There are, for example, the following references related to the optical code division multiplexing communication scheme: (1) “Optical CDMA: Extending the Life of Optical Networks” (Dr. H. Fathallah, APN Inc.), http://www.stanford.edu/˜supriyo/White.pdf; (2) “8-channel Bi-directional Spectrally Interleaved OCDM/DWDM Experiment Employing 16-chip, Four-Level Phase Coding Gratings, ” OECC2002 (P.C. Teh et. al, OECC2002 Technical Digest 11A-1, p384–38); and (3) “Multiple-Phase-Shift Superstructure Fiber Bragg Gratings (MPS-SSFBG's) for Dense WDM Systems,” Nasu et al, OECC/IOOC2001, PDP1.
However, there has been no example in which encoded signals of the same wavelength are multiplexed strictly in the same time slot. For example, it is clearly described that with encoded signals using a coherent light source, signals spread in the time direction or time axis cannot be overlapped with one another. For example, see Reference (1) supra. Also, Reference (2), for example, discloses a multiplexing transmission which uses SSFBG (Super Structure Fiber BraggGrating) based phase encoded signals. However, Reference (2) does not clearly demonstrate the structure of SSFBG except for the overall length, and characteristics except for reflection spectrum. Also, in Reference (2), the multiplexing transmission uses the WDM technology in combination, wherein a plurality of phase encoded signals at different wavelengths are multiplexed in a time domain, whereas a duration time for the encoded signals is set identical to a data period. In other words, encoded signals of the same wavelength, spread in the time axis, are not overlapped with one another, as disclosed in Reference (1).
Therefore, when signals spread in the time axis are overlapped with one another, interference among optical pulses can damage the transmission characteristics, limit the data rate, transmission distance and the like, and also causes other problems. Also, if encoded signals of the same wavelength cannot be multiplexed in the same time slot as described above, an upper limit for a data rate applicable in an optical communication system is determined by an encoder, so that the flexibility of the optical communication system is limited by the encoder.
The SSFBG disclosed in Reference (2), on the other hand, cannot be applied to a system at a data rate or higher having a data period equal to or less than a duration time (total duration) determined by the length of the encoder. For example, when the duration time is 800 ps, the SSFBG cannot be applied to a data rate of 1.25 Gbps (Gigabits per second) or higher. Further, it is effective to increase the number of code chips for increasing the number of codes for multiplexing using the SSFBG. However, a simple increase in the number of code chips would result in an increase in the length of the SSFBG, thereby further limiting the data rate to which the SSFBG can be applied.